Consumers value the ability to see the contents of their packages. Consumers also appreciate the toughness and gloss of containers made from polyester. Because of this combination of attributes, containers made from polyethylene terephthalate (PET) produced by the injection stretch blow molding process (ISBM) are the most common type of transparent container on the market. However, the ISBM process is limited to uniform shapes and cannot produce bottles that contain a through-handle. Handles are desirable in larger bottle sizes, where gripping a round or square container becomes cumbersome. Larger size bottles containing a through handle are believed to be produced only by the extrusion blow molding (EBM) process.
A typical extrusion blow-molding manufacturing process involves: 1) melting the resin in an extruder; 2) extruding the molten resin through a die to form a tube of molten polymer (i.e. a parison); 3) clamping a mold having the desired finished shape around the parison; 4) blowing air into the parison, causing the extrudate to stretch and expand to fill the mold; 5) cooling the molded article; 6) ejecting the article of the mold; and 7) removing excess plastic (commonly referred to as flash) from the article.
The hot parison that is extruded in this process often must hang for several seconds under its own weight prior to the mold being clamped around it. During this time, the extrudate must have good molten dimensional stability, also known as melt strength. Melt strength is directly related to the viscosity of the material at low shear rates, such as 1 sec−1. A material with good melt strength (i.e. high viscosity) can resist stretching and flowing (a.k.a. sag) that would cause uneven material distribution in the parison and thinning of the parison walls. The sag of the extruded parison is directly related to the weight of the parison, whereby larger and heavier parisons will have a greater tendency to sag. Heavier parisons are required as bottle size increases, whereby the production of larger bottles requires higher melt strength. Materials with high melt strength will also resist tearing while the parison is blown into a bottle. Thus, good melt strength is required to form good quality containers, particularly those of larger size, that have uniform side wall thickness and that will not tear during expansion (i.e. blowing).
The two types of extrusion blow molding that involve a hanging parison are referred to as “shuttle” and “intermittent” processes. In a shuttle process, the mold is situated on a moving platform that moves the mold up to the extruder die, closes it around the parison while cutting off a section, and then moves away from the die to inflate, cool and eject the bottle. Due to the mechanics of this process, the polymer is continuously extruded through the die at a relatively slow rate. By contrast, the mold in an intermittent process is fixed below the die opening and the full shot weight (the weight of the bottle plus flash) of polymer must be rapidly pushed through the die after the preceding bottle is ejected but before the current bottle is inflated. Intermittent processes can either utilize a reciprocating screw action to push the parison, or the extrudate can be continuously extruded into a cavity which utilizes a plunger to push the parison.
In a very different type of extrusion blow molding process, a 4-20 ft diameter wheel moving at 1-10 revolutions per minute grabs the parison as it extrudes from the die and lays it in molds attached to the wheel's outer circumference. Mold close, parison inflation, cooling and ejection of the bottle occurs sequentially as the wheel turns. In this “wheel process”, the parison is actually pulled from the die by the wheel whereby good melt strength is required to prevent thinning of the parison during both pulling as well as subsequent blowing. The parison in a wheel process can exit the die in either an upward or downward direction and melt strength will be more crucial during upward extrusion due to the effects of gravity. Because of the continuous nature of this “wheel” process, polymer can be extruded from the die at very high speeds.
Unfortunately, extrusion at high speed which sometimes occurs during wheel and intermittent extrusion blow molding processes can create a condition known as “sharkskin” on the surface of the extruded part or article. Sharkskin (a form of melt fracture) is visually observable as a frosty white matte surface haze, and is an undesirable defect in transparent bottles. Sharkskin is a rheological flow instability phenomenon that occurs as molten polymer flows at high shear rates over a metal surface, such as the surface of the extruder die. Shear rates between 100 sec−1 and 1000 sec−1 are typically generated at the die due to the need to obtain reasonably fast production rates while simultaneously generating thin walls for lightweight bottles. Wheel and Intermittent processes create the highest shear rates.
Sharkskin can be avoided by increasing the process melt temperature, which lowers the material's viscosity, but this also leads to a reduction in melt strength. Nonetheless, the shear rate associated with melt strength is typically only around 1 sec−1. Thus, a material with both good melt strength (i.e. high viscosity) at low shear rates and resistance to sharkskin (i.e. low viscosity) at high shear rates is highly desirable for extrusion blow molding. This behavior is referred to as shear thinning.
The typical PET resins used to ISBM beverage containers are believed to be difficult to extrusion blow mold due to their relatively low inherent viscosities (IV≦0.90 dL/g) and high crystalline melting points (>245° C.) which leaves them with low melt strength at the temperatures needed to process them. These ISBM PET resins can be further solid stated to increase viscosity, but these compositions still do not have sufficient shear thinning behavior to prevent sharkskin. Numerous attempts have been made to add branching agents to PET to improve the shear thinning characteristics, but these compositions are believed to require solid stating. Solid stated branched PET compositions are particularly prone to issues with gels and unmelts during the EBM process.
In order to overcome these problems, U.S. Pat. No. 4,983,711 describes totally amorphous copolyester compositions related to PET that are particularly useful in extrusion blow molding processes. These compositions comprise terephthalic acid or DMT moieties with ethylene glycol and 25-75 mole % 1,4-cyclohexanedimethanol and 0.05 to 1 mole % of a branching agent. These compositions are particularly desirable for extrusion blow molded beverage containers since they yield containers with clarity, gloss and toughness similar to ISBM PET containers.
Unfortunately, containers made from compositions described by U.S. Pat. No. 4,983,711 can cause problems in the PET recycle stream. Ground flake from these containers can stick to the walls of the dryer or agglomerate with PET container flake in a dryer set at 140-180° C. Mixing ground flake from these containers into PET container flake could also result in hazy film, sheet or bottles. It is possible to sort out the compositions described in U.S. Pat. No. 4,983,711 from the PET recycle stream, but a much more desirable solution is to find a material that can be both extrusion blow molded into transparent containers, but will be non-problematic in the PET recycle stream at levels much higher than they will be present in the recycle stream.
In addition, copolyesters of the compositions described in U.S. Pat. No. 4,983,711 can have high levels of sharkskin problems when processed on high output processing equipment such as wheel machines.
Sharkskin can also be a problem in profile extrusion. Profile extrusion is a common, cost-effective method for producing shaped articles. Profiles can take on a wide variety of cross-sections varying in size, shape and complexity. Common “simple” profile shapes include hollow tubes, solid round stock, square cross-section stock, etc. More complex shapes such as those used for pricing channels, corner guards, and house siding can also be made. Profiles are fabricated by melt extrusion processes that begin by extruding a thermoplastic melt through an orifice of a complex die thereby forming an extrudate capable of maintaining a desired shape. The extrudate is typically drawn into its final dimensions while maintaining the desired shape and then quenched in air or a water bath to set the shape, thereby producing a profile. In the formation of simple profiles, the extrudate preferably maintains shape without any structural assistance. With extremely complex shapes, support means are often used to assist in shape retention. In either case, the type of thermoplastic resins utilized and its melt strength during formation is critical. For example, when extruded vertically from a die, a polymer with low melt strength will quickly sag and hit the floor; whereas, a polymer with high melt strength will maintain its shape for a much longer amount of time.
Inadequate melt strength results in severe processing problems when polyesters are processed at typical profile extrusion line speeds and temperatures of 390-550° F. (200-290° C.). Process line speeds vary considerably depending on the shape of the profile. Typical speeds for simple shapes like a corner guard may be from 50 to 70 feet (15 to 20 meters) per minute. More complicated shapes may have process line speeds as low as one foot (0.3 meters) per minute, whereas extremely simple shapes with certain types of processing technology may run at speeds as high as 100 feet (30 meters) per minute. At the higher speeds, which obviously would be preferred by profile manufacturers to reduce cost, inadequate melt strength produces an extrudate that does not maintain its shape prior to quenching, and thus deformation occurs. To increase the melt strength of the polyester, processing temperatures are often lowered. This, however, increases the likelihood of sharkskin, which can only be eliminated by lowering the extrusion speed. By decreasing speed, the economic attractiveness of using polyesters is also decreased. Thus, the profile extrusion processes are often operated at maximum speeds associated with the highest temperatures and minimal melt strengths for maintaining particular profile shapes. Any increase in speed or lowering of temperature may cause an increase in high shear viscosity in the die, which then may cause undesirable sharkskin. The die gap thickness in profile extrusion is often 1 mm or less. Shear-stress at the die land in these cases is most often extreme, thus making the onset of sharkskin quite common.
Thus, there is a need in the art for a transparent material with high resistance to sharkskin in extrusion blow molding and profile extrusion processes. It would also be useful to find a material that can be both extrusion blow molded into transparent containers, but that will also be non-problematic in the PET recycle stream.